break!: make Label::new panic for backward spans

This should help tracking down backward spans and
it's better to have one method that can panic rather
than 4 methods that can panic (Report::write,
Report::write_for_stdout, Report::print, Report::eprint).

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 # Unreleased
 
+### Breaking changes
+
+- Added missing `S: Span` bound for `Label::new` constructor.
+
+- Previously labels with backwards spans could be constructed and
+  only resulted in a panic when writing (or printing) the report.
+  Now `Label::new` panics immediately when passed a backwards span.
+
 ### Added
 
 ### Removed

src/lib.rs

@@ -78,10 +78,19 @@ pub struct Label<S = Range<usize>> {
     priority: i32,
 }
 
-impl<S> Label<S> {
+impl<S: Span> Label<S> {
     /// Create a new [`Label`].
     /// If the span is specified as a `Range<usize>` the numbers have to be zero-indexed character offsets.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the given span is backwards.
     pub fn new(span: S) -> Self {
+        assert!(
+            span.start() <= span.end(),
+            "Label start is after its end"
+        );
+
         Self {
             span,
             msg: None,
@@ -427,3 +436,9 @@ impl Default for Config {
         }
     }
 }
+
+#[test]
+#[should_panic]
+fn backwards_label_should_panic() {
+    Label::new(1..0);
+}

src/write.rs

@@ -42,11 +42,6 @@ impl<S: Span> Report<'_, S> {
                 }
             };
 
-            assert!(
-                label.span.start() <= label.span.end(),
-                "Label start is after its end"
-            );
-
             let start_line = src.get_offset_line(label.span.start()).map(|(_, l, _)| l);
             let end_line = src
                 .get_offset_line(label.span.end().saturating_sub(1).max(label.span.start()))
@@ -884,15 +879,6 @@ mod tests {
         "###);
     }
 
-    #[test]
-    #[should_panic]
-    fn backwards_label_should_panic() {
-        let _ = Report::<Range<usize>>::build(ReportKind::Error, (), 0)
-            .with_label(Label::new(5..0))
-            .finish()
-            .write_to_string(Source::from(""));
-    }
-
     #[test]
     fn label_at_end_of_long_line() {
         let source = format!("{}orange", "apple == ".repeat(100));